FIG. 1 illustrates the basic external topology of an exemplary 3-pin boost converter.
To better understand the context of the present invention it may be helpful to the reader to set forth the basic boost circuit operations of interest in the present embodiment, whereby some simplified schematics of conventional boost operation in the continuous mode and the discontinuous mode are shown in FIGS. 2 and 3, respectively. In particular, FIG. 2 illustrates a simplified view, with the control circuitry and switches omitted, of a conventional model for the two states of typical boost mode converter in the continuous conduction mode of operation.
FIG. 3 illustrates a simplified view, with the control circuitry and switches omitted, of a conventional model for the three states of typical boost mode converter in the discontinuous conduction mode of operation.
FIG. 4 shows the simplified control structure of an exemplary boost regulator, in accordance with an embodiment of the present invention. An output voltage is sensed through a resistor divider RF1 and RF2, and the difference between the divided voltage and a reference voltage is integrated and fed into a summing comparator. An integrated error voltage is summed with a signal indicative of an inductor current as well as a compensating voltage ramp. The output of the summing comparator forms a pulse width modulated control (PWM) signal that turns a large NFET on and off. When the NFET is on, the NFET allows current to ramp up in an external inductor; when the NFET is off, the current in the inductor tends to flow in the same direction, which drives the drain of the NFET high. A commutating PFET device turns on when its drain voltage and the NFET drain and one side of the inductor exceed the voltage at the output. In continuous mode conduction, where the inductor current is never at zero, the PFET and the NFET are alternately in the on position. During discontinuous mode conduction where the inductor current may be zero, both the PFET and the NFET may both be simultaneously in the off position for part of the cycle. Circuitry inside a LOGIC and DRIVERS block prevents the PFET from allowing current to flow backwards from a capacitor Cout into a voltage source Vin through the inductor.
In the circuit shown, as the load current is decreased, the circuit will move from continuous mode operation into discontinuous mode operation and finally, due to the minimum current pulse limit inherent with conventional leading edge blanking, pulses will start to be skipped; this means that a clock edge will occur but the PWM comparator will prevent a new cycle from starting. After a certain number of pulses have been skipped without a new cycle being initiated, and when the integrator circuit indicates that the output is reasonably close to its regulated value or equivalently, if a certain time passes without a new cycle being initiated and the integrator circuit indicates that the output is reasonably close to its regulated value, the circuit can move into a standby operation mode, the STDBY state.
In the standby (STDBY) state all nonessential portions of the circuit are typically shutdown. Typically, essential circuits that stay active include, without limitation, an auxiliary control circuit and a timing circuit. Certain digital circuits may still operate; however, for circuit implemented using CMOS technology, the supply current requirement of these digital circuits is generally almost zero. Accordingly, voltage reference, integrator, feedback resistor, PWM comparator, oscillator, ramp generator and inductor current sense circuitry are all preferably turned off and, as such, require essentially zero supply current in the STDBY state.
Switching regulators, like those described above, are often used in power control circuits for battery operated hand-held devices. These regulators continue to be driven by two forces: low power consumption and small size. Yet these circuits must be able to provide regulated power from no load up to their full rated load at all times. At very light loads, efficiency may drop off because the supply currents used to keep the power control circuits running become a significant portion of current supplied to the load. Since the total power draw during this situation is very low, this may not be perceived as a critical issue. However, this situation is very relevant because these light load operating modes may be active for very long periods of time. Eventually these light load operating modes can consume a large amount of the energy stored in a battery.
Various schemes have been invented over the years that allow switching power supplies to reduce their power consumption during periods of light loading. Many of these schemes work by sending out bursts of switching pulses separated by long periods of inactivity when portions of the circuit can be turned off in order to reduce the current required to keep the control circuit active. It is important that the control circuitry retains enough of its original operation so that the circuitry can respond to a sudden increase in output loading without falling out of regulation. Schemes where the operation is degraded during these special light loading modes, such that the transient response of the circuit allows the output voltage to fall out of regulation, are of questionable value.
While most of these schemes might work well at light load currents most of these schemes are not suitable for situations where the load current requirement is almost zero, for instance, without limitation, where the portable hand-held device is turned off but it still needs to keep a CMOS logic circuit in a particular state without switching so that it can remember its state for the next time the device is turned on. In these situations any supply current that is required by the power control circuitry that is more than the self-leakage current of the battery powering the hand-held device would be too much.
In view of the foregoing, there is a need for a power control circuit that can maintain itself in an operating condition, ready to respond to increased loading at any moment, with a small supply current rating, for example, without limitation, a supply current of less than 1 μA.